The present invention relates to an improved drive shaft coupling device, including an improved shear bolt arrangement for the coupling device.
Overload protection devices, as may be installed on rotating drive shafts, must be able to withstand various loads and conditions. It is important that the overload devices connect both ends or members of the drive shaft in such a manner that there is no relative movement between the drive shaft ends, either axial or radial. Upon encountering an overload condition and the consequent rupturing of the shearing connectors, the overload devices should allow the two drive shaft ends to rotate relative to each other consistently at pre-determined torque loads. Previous is designs have not been able to attain consistency of shearing while maintaining absolute relative proximity (i.e. position or spacing) of the two ends of the drive shaft coupling device. In previous designs, the fact that the relative proximity of the ends of the drive shaft coupling device must be maintained has meant that inconsistent friction between the two ends of the coupling device has resulted in inconsistent shearing of the shear pins or bolts.
Inconsistencies in the friction between the two ends of drive shaft coupling devices have several causes, such as improper torque settings of the shear bolts, inconsistent surface finishes of the mating or contacting surfaces of the drive shaft coupling device, or corrosion between the contacting surfaces of the coupling device. Although in theory these inconsistencies might be overcome with conventional shear bolts or pins, in reality it is difficult and impractical.
The novel shear bolt of the present invention, designed in conjunction with novel flanges of the drive shaft coupling device, provide a relative positioning or spacing of the two ends of the drive shaft coupling device, yet allow a consistently controllable shearing of the device in overload conditions. The spacing of the two ends of the drive shaft coupling device is maintained both before and after overload. The designed or desired shear torque of the new drive shaft coupling device is not affected by inconsistencies in the torque settings of the shear bolt nuts, inconsistencies of surface finish in the opposing ends of the drive shaft coupling device, nor by corrosion of the mating or contacting surfaces of the device. With the novel design of the present invention, a desired result is attained consistently and economically.
The invention provides a device to connect the opposed flanges of a drive shaft coupler in a manner so as to maintain the relative position or spacing of the opposed flanges, and yet allows for consistent and controllable shearing of the connecting shear bolts upon the drive shaft coupler encountering an overload condition.
The present device has a first drive shaft coupler flange and a second drive shaft coupler flange rotatable about a common axis. Each flange has at least two shear bolt retaining slots which may be axially aligned with corresponding shear bolt retaining slots on the other opposing flange.
The device has at least two shearable connectors or bolts, each one threaded from both ends with a shoulder at the end of the threads. Between the two shoulders there is a shear point or area that is turned down on a lathe in order to be of a closely controlled diameter and to be of a lesser shear strength than the rest of the shear bolt. The connectors are to be arranged for balanced common rotation of the first and second drive shaft coupler flanges, and when the connectors shear or rupture upon overload condition of the drive shaft coupler.
The shoulders of shearable connectors provide a means of controlling the distance between the two opposing flanges of the drive shaft coupler as well as providing a surface with which to oppose the tension applied to the shearable connector when tightening the nut on the threads of the connector in order to rigidly affix the connector to the drive shaft coupler flange.
The shearable connectors also have opposing flat surfaces located between the end of the threads and the shoulders of the connector, which flat surfaces fit closely inside the shearable connector retaining slots of the drive shaft connector in order to prevent the shearable connector from rotating when installing or removing the nuts from the ends of the connectors.
The drive shaft coupler flanges also have a circular recessed area on the exposed side of the flange where the nut of the shearable connector contacts the flange. The circular recessed area is of a size to fit a standard flat washer which is in turn the appropriate size to fit the connector. A purpose of the recessed area is to locate the connector at the desired distance from the axial center of the drive shaft coupler.
The device also has a means to seal contaminants and corrosive elements out of the area of the drive shaft coupler where metal parts are in close proximity to each other, and a means to periodically introduce lubrication into the sealed area.